This invention relates to a process for making necked nonwoven webs and laminates having more uniform basis weights and stretching properties, and to necked nonwoven webs and laminates so made.
Necked nonwoven webs, including necked spunbond webs, meltblown webs, combinations and the like, are often made using a process which is schematically illustrated in FIG. 1. A nonwoven web 12 having a starting width A is passed in its machine direction between a first nip 16, which can be a first pair of nip rollers traveling at a first surface velocity, and a second nip 26, which can be a second pair of nip rollers traveling at a second surface velocity which is faster than the first surface velocity. The surface velocity difference between the first and second nips results in formation of a narrower (xe2x80x9cneckedxe2x80x9d) nonwoven web 22 having a necked width Axe2x80x2 which is less than the starting width A.
The necked nonwoven web 22 generally includes fibers which are closer together and more aligned in the machine direction than the fibers of the starting nonwoven web 12, which can be more randomly aligned. The necking may be performed with the aid of heat applied below the melting temperature of the fibers, for instance, by placing an oven or other heat source between the first and second nips. The necked nonwoven web 22 may also be heat set, either during or after the necking process, so that the necked web becomes somewhat stable. A nonwoven web which is stable in the necked condition is said to be xe2x80x9creversibly neckedxe2x80x9d. A reversibly necked nonwoven web can be easily extended in the cross direction by applying a small extension force, and tends to return to its narrower, necked configuration when the extension force is released.
The starting nonwoven web 12 includes edge regions 13 and 15, and a central region 11. The necked nonwoven web 22 includes edge regions 23 and 25, and a central region 21. Because the necking causes the nonwoven fibers to become closer together and more aligned, without noticeably stretching or narrowing the individual fibers, the necked nonwoven web 22 generally has a higher basis weight than the starting nonwoven web 12.
As can be easily seen from FIG. 1, the nonwoven fibers in the edge regions 13 and 15 of the starting nonwoven web travel a greater distance between the first nip 16 and the second nip 26 of the necking process, than the fibers in the central region 11. Furthermore, the cross-directional stresses in the central region 11 are at least partially counteracted, because these stresses are applied in both cross directions. The cross-directional stresses in each of the edge regions 13 and 15 are primarily in one direction, inward toward the center of the web. This results in increased fiber gathering and necking in the edge regions. Consequently, the fibers in the edge regions 23 and 25 of the necked nonwoven web are generally more aligned and closer together than the fibers in the central region 21. As a result, the necked nonwoven web may be nonuniform in the cross direction, having a higher basis weight in both edge regions than in the central region, and having greater cross-directional extendibility in both edge regions than the central region.
There is a need desire for a necking process which produces necked nonwoven webs having better cross-directional uniformity. There is also a need or desire for necked nonwoven webs, and laminates containing necked nonwoven webs, which have better cross-directional uniformity.
As used herein, the term xe2x80x9crecoverxe2x80x9d refers to a contraction of a stretched material upon termination of a biasing force following stretching length of the material by application of the biasing force. For example, if a necked material having a relaxed, unbiased width of one (1) inch is elongated 50 percent in the cross direction by stretching to a width of one and one half (1.5) inches the material would be elongated 50 percent (0.5 inch) and would have a stretched width that is 150 percent of its relaxed width. If this exemplary stretched material is relaxed, and is recovered to a width of one and one tenth (1.1) inches after release of the biasing and stretching force, the material would have recovered 80 percent (0.4 inch) of its one-half (0.5) inch elongation. Recovery may be expressed as [(maximum stretched dimension minus final sample dimension)/(maximum stretched dimension minus initial sample dimension)]xc3x97100.
As used herein, the term xe2x80x9cnonwoven webxe2x80x9d means a web that has a structure of individual fibers of threads which are interlaid, but not in an identifiable repeating manner. Nonwoven webs have been, in the past, formed by a variety of processes such as, for example, meltblowing processes and bonded carded web processes.
As used herein the term xe2x80x9cmicrofibersxe2x80x9d means small diameter fibers having an average diameter not greater than about 100 microns, for example, having a diameter of from about 0.5 microns, more specifically microfibers may also have an average diameter of from of from about 4 microns to about 40 microns.
As used in herein, the term xe2x80x9cinterfiber bondingxe2x80x9d means bonding produced by thermal bonding or entanglement between the individual nonwoven fibers to form a coherent web structure. Fiber entangling is inherent in the meltblown processes but may be generated or increased by processes such as, for example, hydraulic entangling or needle punching. One or more thermal bonding steps are employed in most processes for forming spunbond webs. Alternatively and/or additionally, a bonding agent can be utilized to increase the desired bonding and to maintain structural coherency of the web. For example, powdered bonding agents and chemical solvent bonding may be used.
As used herein, the term xe2x80x9cmeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high velocity gas (e.g. air) stream which attenuates the filaments of molten thermoplastic material to reduce their diameters, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin, the disclosure of which is hereby incorporated by reference.
As used herein, the term xe2x80x9cspunbonded fibersxe2x80x9d refers to small diameter fibers which are formed by extruding a molten thermoplastic material as filaments from plurality of fine, usually circular, capillaries in a spinnerette with the diameter of the extruded filaments then being rapidly reduced, for example, by eductive drawing or other well-known spun bonding mechanisms. The production of spun-bonded nonwoven webs is illustrated in patents such as, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al. The disclosures of both these patents are hereby incorporated by reference.
As used herein, the term xe2x80x9cnecked materialxe2x80x9d refers to any material which has been constricted in at least one dimension by processes such as, for example, drawing or gathering.
As used herein, the term xe2x80x9cneckable materialxe2x80x9d means any material which can be necked.
As used herein, the xe2x80x9ccentral regionxe2x80x9d of a nonwoven web is defined as the central 70% of the cross-directional width of the nonwoven web. The xe2x80x9cedge regionsxe2x80x9d are defined as the outermost 15% of the width on both sides of the central region of the nonwoven web.
As used herein, the term xe2x80x9creversibly necked materialxe2x80x9d refers to a necked material that has been treated while necked to impart memory to the material so that, when a force is applied to extend the material to its pre-necked dimensions, the necked and treated portions will generally recover to their necked dimensions upon termination of the force. One form of treatment is the application of heat. Generally speaking, extension of the reversibly necked material is substantially limited to extension to its pre-necked dimensions. Therefore, unless the material is elastic, extension too far beyond its pre-necked dimensions will result in material failure. A reversibly necked material may include more than one layer, for example, multiple layers of spunbonded web, multiple layers of meltblown web, multiple layers of bonded carded web or any other suitable combination or mixtures thereof, as described in U.S. Pat. No. 4,965,122, which is incorporated by reference.
As used herein, the term xe2x80x9cpercent neckdownxe2x80x9d refers to the ratio determined by measuring the difference between the pre-necked dimension (width) and the necked dimension (width) of a neckable material and then dividing that difference by the pre-necked dimension of the neckable material.
As used therein, the term xe2x80x9cpercent stretchxe2x80x9d refers to the ratio determined by measuring the increase in the stretched dimension (in any direction) and dividing that value by the original dimension (in the same direction), i.e., (increase in stretched dimension/original dimension)xc3x97100.
As used herein, the term xe2x80x9ccomposite elastic necked bonded materialxe2x80x9d refers to a material having an elastic sheet joined to a necked material at least at two places. The elastic sheet may be joined to the necked material at intermittent points or may be completely bonded thereto. The joining is accomplished while the elastic sheet and the necked material are in juxtaposed configuration. The composite elastic necked-bonded material is elastic in a direction generally parallel to the direction of neckdown of the necked material and may be stretched in that direction to the breaking point of the necked material. A composite elastic necked-bonded material may include more than two layers. For example, the elastic sheet may have necked material joined to both of its sides so that a three-layer composite elastic necked-bonded material is formed having a structure of necked material/elastic sheet/necked material. Additional elastic sheets, necked material layers, and/or inherently extendible materials such as bonded carded webs may be added. Other combinations of elastic sheets and necked materials may be used, for instance, as indicated in U.S. Pat. No. 5,336,545, which is incorporated by reference.
As used herein, the term xe2x80x9cpolymerxe2x80x9d generally includes, but is not limited to homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term xe2x80x9cpolymerxe2x80x9d shall include all possible geometrical configurations of the material. These configurations include, but are not limited to, isotactic, syndiotactic and random symmetries.
As used herein, the term xe2x80x9cselectivelyxe2x80x9d encompasses the terms xe2x80x9conlyxe2x80x9d and xe2x80x9cto a greater extentxe2x80x9d.
As used herein, the term xe2x80x9cconsisting essentially ofxe2x80x9d does not exclude the presence of additional materials or process steps which do not significantly affect the desired characteristics of a given composition or product. Exemplary materials of this sort would include, without limitation, pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters, solvents, particulates and materials added to enhance processability of the composition.
As used herein, the term xe2x80x9ccomprisingxe2x80x9d opens the claim to inclusion of additional materials and/or process steps other than those recited.
The present invention is directed to a process for making a necked nonwoven web having better cross-directional uniformity, and to necked nonwoven webs and laminates so made. As with the conventional process illustrated in FIG. 1, the process of the invention includes the steps of passing a nonwoven web through a first nip having a first average surface velocity, and a second nip having a second average surface velocity which is higher than the first average surface velocity, and necking the nonwoven web between the nips. The second average surface velocity is about 1.05-1.7 times the first average surface velocity, suitably about 1.1-1.5 times the first average surface velocity desirably about 1.2-1.4 times the first average velocity.
As explained above, the conventional necking process inherently necks the edge regions of the nonwoven web to a greater extent than the central region. Therefore, to achieve a more uniform necking profile, the process of the invention further includes the step of varying the necking in the central region relative to the two edge regions. The varying may involve increasing the necking in the central region of the nonwoven web relative to the edge regions, and/or decreasing the necking of the edge regions relative to the central region. This is accomplished by reducing the necking resistance of the central region relative to the edge regions, increasing the necking resistance of the edge regions, increasing the necking force applied to the central region, and/or reducing the necking force applied to the edge regions.
In one embodiment of this invention, the necking resistance of the central region of the nonwoven web relative to the edge regions is reduced by selectively heating the central region. The selective heating can be accomplished either by a) heating the central region only, and not the end regions, or b) heating the entire nonwoven web, but heating the central region to a higher temperature and/or over a longer distance than the end regions. The selective heating should not be so great as to melt any of the nonwoven fibers. The selective heating reduces necking resistance in the central region (effectively increasing the necking in that region) by rendering the nonwoven fibers in the central region softer and more pliable than the nonwoven fibers in the two end regions. Alternatively, the edge regions may be selectively chilled, to increase the necking resistance in the edge regions relative to the central region.
In another embodiment of the invention, the necking force applied to the central region of the nonwoven web is selectively increased by increasing the distance that the central region travels between the first and second nips, relative to the distance traveled by the two edge regions. This can be accomplished by passing the nonwoven web around a profiled guide roller located between the first and second nips in the necking process. The profiled guide roller has a central portion and two end portions. The central portion has a larger diameter than the two end portions. Each turn of the profiled guide roller pulls the central region of the nonwoven web more than the edge regions. The central region of the nonwoven web, which passes over the central portion of the guide roller, experiences a corresponding increase in distance traveled and necking force relative to the two edge regions. The resulting necked nonwoven web has more uniform basis weight and cross-directional extendibility across its width. The necked nonwoven web may be heat set to preserve the necking.